refactor(build): remove INCLUDE_GENERATED_DECLARATIONS guards

These are not needed after #35129 but making uncrustify still play nice
with them was a bit tricky.

Unfortunately `uncrustify --update-config-with-doc` breaks strings
with backslashes. This issue has been reported upstream,
and in the meanwhile auto-update on every single run has been disabled.

src/nvim/spellfile.c

@@ -529,9 +529,7 @@ typedef struct {
   int si_newcompID;             // current value for compound ID
 } spellinfo_T;
 
-#ifdef INCLUDE_GENERATED_DECLARATIONS
-# include "spellfile.c.generated.h"
-#endif
+#include "spellfile.c.generated.h"
 
 /// Read n bytes from fd to buf, returning on errors
 ///